Ko Higashitani

Structural changes in protein molecules adsorbed on ultrafine silica particles

The structural changes in horse cytochrome c, bovine ribonuclease A (RNase A), sperm whale myoglobin, ovalbumin, human hemoglobin, and bovine serum albumin (BSA) during adsorption on ultrafine silica particles have been studied. Since the intensity of light scattered by the ultrafine silica particles (average diameter 15 nm) is negligible, the structure of adsorbed protein molecules on the surface is directly estimated by measuring the circular dichroism (CD) spectra of the suspension of particles on which protein molecules were adsorbed. While the CD spectra of soft proteins such as hemoglobin and BSA were changed extensively by adsorption on ultrafine silica particles, those of rigid proteins such as cytochrome c and RNAse A were changed little. Changes in the CD spectrum of BSA during adsorption increased with decreasing pH. Thus, the magnitude of the structural changes is affected by both the flexibility of the protein molecules and the affinities of the particles for proteins. The BSA molecules desorbed from the ultrafine silica particles by addition of morpholine showed CD spectra similar to those of native BSA and hence were refolded. Thus, the conformational changes in BSA molecules produced by adsorption were highly reversible. The adsorption amounts of all these proteins on ultrafine silica particles were maximum at around their isoelectric points regardless of structural adaptability.

Adsorption of model proteins with wide variation in molecular properties on colloidal particles

To elucidate some general principles of protein adsorption, the adsorption isotherms of ribonuclease A (RNase A), cytochrome c, lysozyme, α-lactalbumin, ovalbumin, and bovine serum albumin (BSA) on colloidal particles of polystyrene, styrene/2-hydroxyethyl methacrylate, and silica were measured as a function of pH and ionic strength. These proteins were different, especially with respect to molecular flexibility. All adsorption isotherms showed plateaus. While affinities of all proteins for hydrophilic particles decreased significantly with increasing pH, those for hydrophobic particles were high irrespective of pH. At an ionic strength of 0.01, the pH dependence of the plateau adsorption of small proteins (RNase A, cytochrome c, lysozyme, and α-lactalbumin) was affected by the surface properties of the particles, while that of large proteins (ovalbumin and BSA) showed the maximum at around their isoelectric points irrespective of the type of particles. On the other hand, the pH dependence of the plateau adsorption was related to the surface properties at an ionic strength of 0.1 irrespective of the type of protein. The above dependence of protein adsorption on properties of solid surfaces and adsorption conditions did not vary with the molecular flexibility of the proteins, although the structural changes in the flexible protein molecules upon adsorption promoted adsorption. Therefore, both the protein-surface interactions, such as hydrophobic and electrostatic ones, and the lateral interaction between adsorbed molecules are dominant factors that control protein adsorption on these particles.

Effects of magnetic fields on stability of nonmagnetic ultrafine colloidal particles

Abstract Effects of exposure to a magnetic field on the stability of nonmagnetic colloidal particles, such as ultrafine polystyrene latex and SiO2 particles in electrolyte solutions, were examined by measuring the rapid coagulation rate constant on a low-angle light-scattering apparatus. It was found that the rapid coagulation rate does depend on the magnetic flux density and the duration of magnetic exposure, even though the magnetic flux density is not high, and that the degree of the magnetic effect depends on the particle size and ions in the medium. It is especially interesting to find that the magnetic effect remains for at least 143 h after the magnetic exposure is completed. It is postulated that these effects are mainly attributable to some alteration of the structure of water molecules and ions adsorbed on the particle surface with the magnetic exposure.

Kinetic and circular dichroism studies of enzymes adsorbed on ultrafine silica particles

Negatively charged ultrafine silica particles (average diameter 20 nm) were used as support materials for adsorption immobilization of porcine trypsin, horseradish peroxidase, and bovine catalase under various conditions, and the changes in the enzyme activities and the circular dichroism (CD) spectra of these enzymes upon adsorption were measured. Since the light scattering intensity of the ultrafine particles was very low, the activities and the CD spectra of the enzymes adsorbed on the particle surfaces could be measured. The enzymes adsorbed at pH around and above their isoelectric points (pI) showed high activities. On the other hand, the enzymes adsorbed at pHs below their pI had significantly diminished activities and showed large CD spectral changes upon adsorption. The extent of CD spectral changes in the enzymes upon adsorption correlated very closely with that of the activity reduction. Therefore, the conformational changes in enzymes upon adsorption are one of the important factors that reduce the activities of adsorbed enzymes. These results demonstrate that the ultrafine particles are not only a novel support for enzyme immobilization but also are helpful for the molecular understanding of the immobilized enzymes.

Effect of particle size on coagulation rate of ultrafine colloidal particles

Abstract The Brownian coagulation rates of nearly monodispersed particles in aqueous solutions with diameters down to about 5 nm were measured with a low-angle light-scattering apparatus to clarify the features of the stability of ultrafine particles. It is found that the rapid coagulation rate obeys the modified Smoluchowski theory when particles are greater than about 0.1 μm in diameter. But the rate reduces abruptly with decreasing particle size when particles are sufficiently small, say smaller than 90 nm in diameter. Two possible mechanisms were considered for this reduction in coagulation rate: (1) particles are deflocculated because of the shallow primary minimum of the interparticle potential which is attributed to the layer of water molecules and ions adsorbed on the particle surface, and (2) the colliding velocity of particles is reduced considerably because of the existence of a layer of high viscosity around the particle surface.

The use of optic fiber probes for the measurement of dilute particle concentrations: calibration and application to gas-fluidized bed carryover

Abstract Dilute particle concentrations are detected by optic fiber probe with a core diameter in excess of the particle diameter. After calibration of the probe, its application to freeboard concentrations is checked. The calibration curve of the probe is determined by detecting known concentrations of free-falling particles, at terminal velocity. The carryover stream of a gas-fluidized bed is thereafter simultaneously probed and collected in a cyclone separator. The linear relationship between probe signal and concentration and the good freeboard correlation obtained, confirms the usefulness of the technique.

Conformational changes in protein molecules upon adsorption on ultrafine particles

Abstract Conformational changes in hemoglobin (a “soft” protein) and catalase (a relatively “soft” protein) upon adsorption on ultrafine silica particles (average diameter 15 nm) have been studied by circular dichroism (CD). Because the light scattering intensity of the ultrafine silica particles is very low, the secondary structure of protein molecules on solid surfaces can be estimated from the CD spectra of the suspension of the ultrafine silica particles on which proteins are adsorbed. The extent of the changes in the secondary structure of both the protein molecules was found to increase with decreasing pH and adsorbed amount. Hemoglobin showed the changes in the secondary structure up to high pH and high adsorbed amount. Therefore, soft proteins are susceptible to conformational changes upon adsorption on solid surfaces. These CD spectrum data give us some quantitative information about the conformational changes in protein molecules upon adsorption on solid surfaces.

Pelleting flocculation of colloidal latex particles

Abstract Experimental conditions required for the formation of pelleted flocs from suspensions of polystyrene latex particles using cationic polymers are examined. It is found that the formation of pelleted flocs depends on the polymer and particle concentrations, the polymer molecular weight and charge density, and the intensity of mixing, and that pelleted flocs are formed only in the limited region where these factors are present, otherwise either microflocs or bulky flocs are formed. It is presumed that the bare surface of the particles and the free cations of the adsorbed polymers on the particle surface which are not consumed by adsorption play an important role in pelletization. The critical volume fraction of particles opt above which pelleted flocs are formed using macroscopically neutralized particles is found to be estimated by the following correlation: opt ≈4 × 10−5 C p N A pN 0 M where Cp is the polymer concentration, NA the Avogadro constant, N0 the initial particle concentration, p the charge density of the polymer, and M the molecular weight of the polymer.

Floc breakup along centerline of contractile flow to orifice

Abstract The breakup process of flocs on the centerline in converging flow to an orifice is observed directly by stroboscopic photographs, using flocs which are composed of a small number of visible particles. An axial symmetric test section is employed to observe the floc behavior before and after the orifice and a two-dimensional one is used to observe flocs within the orifice. It is found that flocs are broken in the velocity acceleration region before the orifice, but not within and after the orifice, and that the maximum size i max of the flocs which experienced the elongation rate (d V z /d Z ) is given by the following equation: i max = 7.0 (d V z /d Z ) −0.2 A model for the breakup of flocs in an elongation flow is proposed, in which the floc breakup is assumed to be attributed to the difference of the drag force on the constituent particles at different locations. It is found that the variations in the size distribution of flocs and the average floc size are successfully predicted by this model.

Axial change of total particle concentration in poiseuille flow

The coagulation of colloidal particles in Poiseuille flow is investigated. An analytical equation for the averaged total particle concentration NE of the solution flowing out of the tube exit in the rapid-coagulation region is derived from quasitheoretical arguments on the coagulation mechanism. The dependence of NE on the axial distance, the flow rate, the particle size, and the initial particle concentration is compared with experimental results obtained for microscopic latex dispersions and it is found that NE of these dispersions is approximately expressed by this equation in the region where the Reynolds number Re < 637. The condition which should be satisfied for the Poiseuille flow coagulation to be negligible is also found.